The present invention relates to a control method for a cycloconverter of the reactive power compensating type in which a power factor of a fundamental wave as seen from a power source side is always 1, and a system for executing the method.
A cycloconverter is a frequency converter for directly converting an AC power at a fixed frequency into another AC power at another different frequency. In the cycloconverter generally constructed by using thyristors, the thyristors when conducted are commutated by a power source voltage. At this time a large amount of reactive power of the power source is consumed. The reactive power constantly varies in synchronism with a frequency of a load connected to the cycloconverter. These causes increase a capacity of an electric power equipment, so that various adverse effects originating from the variation of the reactive power are applied to various types of electric power equipments connected to the cycloconverter. Many proposals for compensating for the reactive power variation have been made.
FIG. 1 shows a cycloconverter using a typical reactive power compensating device of the prior art. A three-phase AC power from the power source 11 is supplied through a three-phase AC bus 12 to a cycloconverter 13 where it is frequency-converted and is supplied to a load 14. Coupled with the three-phase AC line 12 are a phase advancing capacitor 15, a reactive power compensating device 16 and a control circuit 17. The compensating circuit 16 is comprised of a thyristor bridge circuit 16a and a DC reactor 16b. The control circuit 17 detects voltage and current on the three-phase AC bus 12 by means of a transformer 17a and 17b. The detected signals are supplied to a reactive power arithmetic circuit 17c where a reactive power of the power source is calculated. A signal representing a magnitude of the reactive power obtained, together with a signal representing the current I.sub.O detected at the transformer 17e, is supplied to a phase control circuit 17d. The firing angles of the thyristors in the thyristor bridge circuit 16a are controlled by an output signal from the circuit 17d. The current I.sub.O flowing through the DC reactor 16b is controlled so that the reactive power becomes zero.
FIG. 2 is a vector diagram showing a relationship of a voltage in one phase of the three-phase bus 12 and current flowing through the respective portions in the prior art system. An instantaneous current I.sub.CC flows into the cycloconverter 13 with respect to the power source voltage V.sub.S. An amplitude of the current I.sub.CC and a phase angle .alpha. of it with the source voltage V.sub.S constantly changes in synchronism with an AC current flowing through the load 14. A constant current I.sub.C leading 90.degree. from the power source voltage V.sub.S flows into a phase advancing capacitor 15. At this time, if a lagged current expressed by EQU I.sub.O '=I.sub.C -I.sub.CC .multidot.sin .alpha.
flows from the line 12 into the reactive power compensating device 16, the power source current I.sub.S is in phase with the voltage V.sub.S. Even if the magnitude and the phase angle .alpha. of the current I.sub.CC change, the voltage V.sub.S and the current I.sub.S are always kept in phase by correspondingly controlling the magnitude of the current I.sub.O '=KI.sub.O and the cycloconverter is operated, so that the power factor of the fundamental wave as seen from the power source 11 is always kept at 1.
Since the prior art cycloconverter is provided with the reactive power compensating device having a thyristor bridge circuit comprised of a plurality of thyristor, the overall system of the cycloconverter is expensive.